Question

Los agricultores______toda la siembra. a. creer b. destruyeron c. oyó

Answer 1

B. Destruyeron 
I say B because it makes sense, "los agricultores destruyeron toda la siembra" (The farmers destroy all the planting)

Answer 2

The answer would be B. Destruyeron.

It would not be creer (to believe) because it is not conjugated in the proper form. The proper form would be "creyeron" but it would not make sense to say "The farmers believed"

It would not be oyó because once again it is not conjugated in the proper form. The proper form would be "oyeron" but once again it would not make sense to say "the farmers heard"

I hope this helped :)